Although he was a conservative justice, Mr Kennedy sided with liberals on certain key votes, including abortion and gay marriage.

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Mr Kennedy has sided with liberals on many decisions

Why are lawmakers divided?

The Senate, which votes to confirm Mr Trump's nominee, is currently controlled by Republicans with a slim 51-seat majority. But that could change after mid-term elections in November.

Speaking at the rally in Fargo, North Dakota on Wednesday evening, Mr Trump warned his supporters that Democrats would "rewrite the Constitution any way they want" unless Republicans could quickly confirm a conservative justice.

"We have to pick one that's going to be there for 40 years, 45 years," he said of Mr Kennedy's successor.

When asked by reporters on Wednesday if he would wait until after the mid-term elections to nominate a successor, Mr Trump said: "I think you want to go as quickly as possible", the Associated Press reported.

The long-simmering tensions over Supreme Court picks became a political flash point in 2016 when the Republicans successfully blocked President Barack Obama's nominee, Merrick Garland, to replace the late Justice Anthony Scalia.

At the time, Republicans said it was unfair to confirm a judge during an election year. Democrats are now pointing to that argument in an effort to halt the confirmation of Justice Kennedy's replacement.

Earlier on Wednesday, Mr Schumer in a televised speech on the floor of the Senate called for delaying a confirmation vote until after the mid-terms, adding that "anything but that would be the absolute height of hypocrisy" from Republicans.

"Hopefully we are going to pick somebody who will be as outstanding," Mr Trump told reporters at the White House shortly after Justice Kennedy's retirement was announced.

Vice-President Mike Pence said on Twitter that the president would nominate "a strong conservative, in the tradition of the late Justice Scalia" who would uphold the "God-given liberties" of the constitution.

Mr Kennedy, who is the second-oldest justice on the nine-member US Supreme Court, earned a reputation as a swing vote conservative who supported liberal arguments on key decisions, including the 5-4 rulings that decided same-sex marriage and upheld abortion rights.

As a result, news of his retirement has raised fears among pro-choice groups that access to legal abortions in several states could be under threat.

Mr Trump, during his presidential campaign, vowed to attempt to overturn Roe v Wade, the 1973 decision legalising abortion throughout the US.

"If we put another two or perhaps three justices on, that is really what will happen," Mr Trump said during an October 2016 debate with Hillary Clinton.